Organometallic ruthenium complexes are catalysts for the homogeneous catalytic dehydroxylation of secondary alcohols, specifically diols and polyols, to the corresponding primary alcohols using H2 gas as the stoichiometric reductant.
Alcohols, diols and polyols are industrial compounds that are widely used as polymer monomers, solvents, and reactants for organic synthesis. One method for the preparation of these compounds is through the dehydroxylation of polyols and diols to the corresponding diols and alcohols.
Ruthenium metal is well known as a catalyst for the dehydroxylation of polyols and diols to the corresponding diols and alcohols (U.S. Pat. Nos. 5,426,249 and 5,543,379). However, ruthenium metal as a catalyst shows little or no selectivity between secondary and primary alcohols.
Braca, et al., (J. Organomet. Chem. 1991, 417, 41-49) reported on the use of a ruthenium complex, [Ru(CO)3I3]xe2x80x94, for the dehydroxylation of glycerol and other polyols. This dehydroxylation reaction has distinct disadvantages. First, hydrogen gas and carbon monoxide were used under harsh reactive conditions. Secondly, in addition to the desired dehydroxylation products, various byproducts were obtained in significant amounts.
Chinn et al., (Organometallics 1989, 8, 1824-1826) described the synthesis and spectroscopic properties of the unstable dihydrogen complex [Cp*Ru(CO)2(H2)]+ which was found to decompose to {[Cp*Ru(CO)2]2(xcexc-H)}+OTfxe2x88x92. No catalytic activity of either of these complexes is mentioned.
The present process for the selective dehydroxylation of diols to their corresponding primary alcohols avoids many of the foregoing disadvantages and has the advantages of mild reaction conditions, the absence of expensive, oxygen-sensitive ligands (such as triaryl phosphines, trialkyl phosphines) and nitrogen-based ligands in the catalytic system, and tolerance to acid and water. Furthermore, the process exhibits high regioselectivity, resulting in the nearly exclusive formation of terminal primary alcohols and xcex1,xcfx89-diol or their ethers as the final hydrogenated reaction products.
The invention is directed to a process for the selective dehydroxylation of a secondary alcohol, comprising: contacting the secondary alcohol with hydrogen in the presence of a catalytically effective amount of a catalyst precursor having the formula {[CpRu(CO)2]2(xcexc-H)}+Qxe2x88x92 or {[CpRu(CO)(PRxe2x80x23)]2(xcexc-H)}+Qxe2x88x92 or {[ZRu(L)]2(xcexc-H)}+Qxe2x88x92, wherein Qxe2x88x92 is a non-coordinating or weakly coordinating nonreactive anion; Cp is xcex75xe2x80x94C5R5 wherein R is selected from the group consisting of hydrogen and substituted and unsubstituted C1xe2x80x94C18 alkyl groups, where any two adjacent R groups can together form a ring; Rxe2x80x2 is selected from the group consisting of hydrogen, alkyl, aryl, alkoxy and aryloxy; Z is hydridotris(3,5-dimethylpyrazolyl)borate or hydrocarbyl hydridotris(3,5-dimethylpyrazolyl)borate; L is (CO)2 or COD; and COD is 1,5-cyclooctadiene.
Preferably, the secondary alcohol additionally contains a primary alcohol functionality. Most preferred is where the secondary alcohol is selected from the group consisting of 1,2-propanediol, glycerol and 1-phenyl-1,2-ethanediol.
In a preferred embodiment, Qxe2x88x92 is OSO2CF3xe2x80x94 or BF4xe2x80x94, and R is hydrogen, methyl, i-propyl, benzyl, dimethylsilyl, or together with the cyclopentadienyl group forms an indenyl ring; Rxe2x80x2 is methyl, phenoxy, p-fluorophenyl, or cyclohexyl; and Z is hydridotris(3,5-dimethylpyrazolyl)borate.
Another embodiment additionally comprises the presence of added H+Qxe2x88x92, preferably HOTf. In another embodiment, added H+Qxe2x88x92 is not present and the secondary alcohol has an aryl group on the carbon containing the secondary alcohol functionality.